Table of Contents
Classification of Gypsum Products
Gypsum products are widely used in dentistry for making impressions, models, dies, and casts. According to the American Dental Association (ADA) Specification No. 25, dental gypsum products are classified into five types based on their physical properties such as compressive strength, setting expansion, and intended clinical use. This classification helps clinicians select the appropriate material for specific dental applications.
Type I: Impression Plaster
- Chemical Form: Calcium sulfate β-hemihydrate
- Physical Characteristics: Lowest compressive strength (~5.2 MPa), low cost, brittle, and soluble in water
- Setting Expansion: Approximately 0.06%
- Water/Powder Ratio: ~0.50 ml/g
- Uses: Historically used for making preliminary dental impressions and bite registrations; rarely used now due to limited elasticity and brittleness
"Impression plaster (Type I) was originally developed for registering occlusal relationships but is seldom used today because more elastomeric materials are preferred." — Phillips
"Impression plaster is a rigid, brittle material and is usually replaced by modern impression materials." — Manappallil
Type II: Model Plaster
- Chemical Form: Calcium sulfate β-hemihydrate
- Physical Characteristics: Porous particles, low compressive strength (~8.8 MPa), economical
- Setting Expansion: Approximately 0.30%
- Water/Powder Ratio: 0.45--0.50 ml/g
- Uses: Ideal for study casts, mounting casts on articulators, and diagnostic models where high strength is not critical
"Model plaster (Type II) is mainly used for study models and for mounting casts." — Phillips
"Type II plaster is used mainly for making study casts and for mounting casts." — Manappallil
Type III: Dental Stone
- Chemical Form: Calcium sulfate α-hemihydrate
- Physical Characteristics: Denser particles than model plaster, improved compressive strength (~20.7 MPa), moderate setting expansion
- Setting Expansion: Approximately 0.08--0.15%
- Water/Powder Ratio: 0.28--0.30 ml/g
- Uses: Working models for complete dentures, partial dentures, orthodontic appliances, and removable prosthetics
"Dental stone (Type III) has greater strength and less porosity compared to model plaster, making it suitable for working casts." — Phillips
"Dental stone is used where greater strength, hardness, and durability are required such as in the fabrication of dentures and orthodontic models." — Manappallil
Type IV: High-Strength Dental Stone (Low Expansion)
- Chemical Form: Modified calcium sulfate α-hemihydrate
- Physical Characteristics: Dense, cuboidal particles, high compressive strength (~34.5 MPa), low setting expansion (0.05--0.10%)
- Water/Powder Ratio: 0.22--0.24 ml/g
- Uses: Precise dies for crowns, bridges, implant models where dimensional accuracy and high strength are essential
"High-strength dental stone (Type IV) is used for fabricating dies for fixed prosthodontics due to its superior strength and minimal setting expansion." — Phillips
"Type IV dental stone is the hardest and strongest among gypsum products, recommended for master dies for crowns and bridges." — Manappallil
Type V: High-Strength, High-Expansion Dental Stone
- Chemical Form: Modified calcium sulfate α-hemihydrate with additives to increase expansion
- Physical Characteristics: Highest compressive strength (~41.4 MPa), controlled high setting expansion (0.16--0.30%) to compensate for alloy shrinkage in casting
- Water/Powder Ratio: 0.18--0.22 ml/g
- Uses: Dies for metal casting, particularly base-metal alloys with high shrinkage requiring compensating expansion
"Type V stone is especially formulated for use where high expansion is required to offset metal casting shrinkage, such as in base-metal alloys." — Phillips
"High-strength, high-expansion dental stone (Type V) is used for dies where increased setting expansion is beneficial for compensating alloy shrinkage." — Manappallil
Classification based on Setting Time and Strength
Gypsum products can also be classified according to two key physical properties important in clinical practice:
- Setting Time — how fast the material hardens after mixing
- Compressive Strength — the load it can withstand without breaking
The following classification is based on standard dental materials references from Phillips, Manappallil, and Craig.
Types Based on Setting Time
| Type | Setting Time Range | Common Clinical Application | Comments |
|---|---|---|---|
| Impression Plaster (Type I) | Very fast (2-4 min initial, 5 min final) | Historical impressions, bite registration (rare now) | Brittle, low strength, sets rapidly |
| Model Plaster (Type II) | Fast (5-12 min initial, 15-30 min final) | Study casts, mounting | Lovable for quick impressions, low strength |
| Dental Stone (Type III) | Moderate (8-15 min initial, 30-40 min final) | Working models, prosthetics | Good balance of strength and time |
| High-Strength Stone (Type IV) | Moderate to slow (12-20 min initial, 45-60 min or longer final) | Precise dies for crowns/bridges, implant models | Requires longer time for max hardness |
| High-Strength, High-Expansion Stone (Type V) | Moderate (similar to IV, 12-18 min initial) | Investment casting dies | Rapid enough to work clinically despite higher expansion |
Types Based on Compressive Strength
| Type | Approximate Compressive Strength (MPa) | Clinical Significance |
|---|---|---|
| Type I | 4 - 8 | Low strength, unsuitable for stress-bearing parts |
| Type II | 9 - 12 | For study models, not load-bearing |
| Type III | 20 - 25 | For working casts with moderate mechanical stresses |
| Type IV | 30 - 40 | High strength for precision dies and crown work |
| Type V | 40 - 50+ | Highest strength to compensate metal casting shrinkage |
Summary Table of Setting Time and Strength
| Type | Setting Time (min) | Compressive Strength (MPa) | Typical Applications |
|---|---|---|---|
| I | 2-5 | 4-8 | Impressions, bite registration (historical) |
| II | 5-30 | 9-12 | Study casts, mounting |
| III | 8-40 | 20-25 | Working models, dentures, orthodontics |
| IV | 12-60+ | 30-40 | Dies for crowns and fixed prosthetics |
| V | 12-45 | 40-50+ | Investment and casting dies, base-metal alloys |
Notes:
- Setting time can vary according to water-powder ratio, mixing, temperature, and additives.
- Final setting time of Type IV stones may be longer to reach full hardness (up to 24 hours in some cases).
- Strength values quoted represent 24-hour or 1-hour strength tests under controlled lab conditions.
- Clinical practice may modify manipulation to optimize working time and strength.
Classification based on Clinical Use
Classifying gypsum products based on their clinical use involves understanding which type of material is suitable for specific dental applications. Different types of gypsum are selected depending on their strength, accuracy, expansion, and handling characteristics, all aligned with their intended clinical purpose.
1. Impression Materials
- Type I: Impression Plaster
- Traditionally used for preliminary impressions.
- Characteristics: Very brittle, low strength, rapid setting (~2-4 minutes).
- Examples: Historically used in edentulous impressions.
- Note: Now mostly obsolete; replaced by elastic impression materials like alginate and elastomers.
- Type II: Model/Study Plaster
- Purpose: For making diagnostic models and mounting casts.
- Characteristics: Moderate strength (~9 MPa), fast setting (~5-12 minutes).
- Application: Diagnostic casts, articulator mounting, and denture base fabrication.
- Properties: Porous, less resistant to wear and tear; economical.
- Type III: Dental Stone
- Purpose: For fabricating working models, removable partial dentures.
- Characteristics: Higher strength (~20 MPa), slightly slower setting (~8-15 minutes).
- Features: Dense, low porosity, more accurate; used for detailed casts.
- Applications: Crown and bridge models, removable prostheses.
2. Die and Crown & Bridge (C&B) Work
- Type IV: High-Strength Die or Densite
- Purpose: Precise dies for crowns and bridges.
- Characteristics: Very high strength (~34-40 MPa), low expansion (~0.05-0.10%), slower setting (~12-20 minutes).
- Application: For wax pattern investing, high accuracy dies.
- Additional features: Good surface detail reproduction; can withstand multiple casting cycles.
- Type V: High-Strength and Expansion
- Purpose: For casting metals requiring dimensional compensation.
- Characteristics: Highest strength (~45 MPa), high expansion (~0.16-0.30%), similar setting times to Type IV.
- Application: Investment casting, especially for base-metal alloys with significant shrinkage.
- Note: Used for cases where die expansion is necessary for fit accuracy.
3. Specialized and Advanced Uses
| Application | Gypsum Type | Key Features | Comments |
|---|---|---|---|
| Mounting casts | Type II or III | Ease of removal, sufficient strength | Cost-effective, used in articulators |
| Provisional restorations | Type II | Good handling, moderate strength | Temporary models |
| Ceramic and porcelain work | Type III or IV | Dimensional accuracy, surface detail | For porcelain jackets, inlay dies |
| Investment for casting | Type V | High expansion, high strength | Metal casting, base-metal alloys |
Gypsum Selection Tree
A flowchart or decision diagram can help visualize this classification, linking types to their specific uses like:
– Quick setting, low strength → Type I & II
– Accurate, durable → Type III & IV
– High strength + expansion → Type V
Key Takeaways
- Type I is the oldest, least used, for impression registering.
- Type II is widely used for diagnostic models.
- Type III balances strength & accuracy for multiple clinical applications.
- Type IV is the standard for working dies used in crown and bridge procedures.
- Type V is for casting dies with specific expansion needs.
This classification guides the dentist in choosing the right material for each step in restorative and prosthetic dentistry, ensuring precision and longevity of restorations.